Multilayer plastic webs, including both films and slabs, can be manufactured by various multilayer extrusion methods. In extrusion coating, the various layers are superimposed outside the tool. In coextrusion, the method differs by bringing together the individual molten currents within the slot die and bringing together the individual molten currents before they reach the slot die, using an adapter. Coextrusion tools for making flat films for the packaging industry are described in greater detail for example in the journal Kunstoffe, Volume 74, No. 5, Pages 393-397, 1988.
In manufacturing multilayer plastic webs, the problem arising of achieving uniform thickness distribution and layer thickness of the individual molten currents and layers. Usually thickness measurements are conducted on the extruded multilayer plastic web after it has already left the slot die, in order to use the measured values obtained to trigger a change in throughput of raw material, in other words tire volumes of the molten currents. In addition, there is also the opportunity to use such measurement data to modify the extraction rate of flat films or plates from the slot die; see in this connection DE-OS 31 07 701 for example. One possibility for adjusting the flow cross sections of the molten currents coextruded into a main current for a multilayer plastic web in the area of guidance to the adapter is known for example from German Patent 37 41 793.
Another method for regulating the film thickness of flat films manufactured by extrusion likewise utilizes thickness measurement of the flat film, extruded from a slot die or coated thereby, over its width, in order to use the measurement data obtained additionally to heat the slot die in the appropriate areas or additionally to cool it in order to effect a corresponding change in the thickness distribution, i.e. of the molten current; see for example DE-OS 30 06 566.
To the extent that thickness measurement values are used for regulating the layer thicknesses of coextruded multilayer plastic webs as well as single-layer plastic webs, the thicknesses obtained after emerging from the slot die are measured in each case. Particularly in the case of multilayer plastic webs, it is difficult to control the molten currents sufficiently accurately in order to even in multilayer plastic webs being drawn out into very thin packaging films, with the individual layer thicknesses being reduced to 2.mu. in a Chill-roll method located downstream of the slot die. Hence, the goal of the invention is to provide an improved opportunity to regulate the individual layer thicknesses of coextruded multilayer plastic webs, in order to insure uniform thickness distribution even in multilayer plastic webs in which individual layers have relatively limited thicknesses, and especially to avoid gaps resulting from cracks in the layers.